How do you return a hidden panel in the properties
Start by creating (splitting) a new area, then set its type to outliner.


Click and drag the hashed corner overlapping the area you want to shrink in order to make way for the new area.
Click the drop down on the far left of the Header of the newly created area
Select Outliner
(optional)  RMB anywhere on the header and select Header > Flip to Top.




You can read more about the interface system in the manual.